Old methods of fabricating single-mode fiber couplers involve positioning of fibers which have had their claddings stripped away and then surrounding the etched fibers with an index-of-refraction matching liquid. Typically, the amount of cladding which remains is about one-half the core diameter and the length of the etched region is in the neighborhood of two centimeters. Optionally, the fibers can be twisted together, disposed in an index matching fluid and held in place by mechanical fasteners. This approach, while meritoriously providing a somewhat acceptable degree of coupling, is prone to fail if the index matching liquid leaks away, if vibrations separate the twisted fibers or if a combination of the two occurs. An alternate approach, of course, is to align a pair of etched fibers and hold them together by collapsing a glass capillary tube about them, but this too has drawbacks.
Because of the limitations and the disadvantages of both the described methods, they leave much to be desired from a designer's standpoint. An acceptable low-loss stable coupler must have the index of refraction of the surrounding medium exactly match that of the claddings and the relative positioning of the etched fiber regions must be strictly maintained to within a very small (less than one micron) range. The first referred to method relies on purely mechanical means to align the fibers and, consequently, is not stable; when external disturbances such as vibrations and temperature changes occur, they cause movement of the mechanical fasteners and, hence of the fibers themselves, to degrade the coupler's stability. The second referred to method has two obvious limiting requirements, those being, that the mechanical and thermal properties of the capillary tube must match those of the glass fibers. When mismatch occurs, either or both the fibers and capillary tubes will crack and become misaligned as they are heated to and cooled from glass's melting temperature. Additionally, when the glass capillary tubes are expected to serve as a cladding-index matching medium, it is very difficult to match the refractive indices to within 0.1% (needed for acceptable single-mode coupling). Identical temperature coefficient and refractive index matching are nearly an impossibility in a practical sense.
A more recent advance is disclosed by Michael K. Barnoski et al. in their "Fiber Optics Access Coupler" U.S. Pat. No. 4,054,366. Their coupler is intended to be used in single strand multimode fiber systems calling for a variety of optical inputs and outputs. Laser beams are used to fuse couplers in a juxtaposed relationship. The coupling largely is dependent on the distance the cores are separated and the length that the cores are held in their side-by-side positioning. The degree of coupling for any given angle between the fibers when they are joined is dependent on the angle they make at the junction between them and the modes that have been excited in the waveguide. Some modes couple more energy into the access fiber primarily because they make a greater propagation angle with respect to the access coupler. The core diameters are in the range of thirty to one-hundred microns and the overlying claddings are between twelve to one-hundred microns. These dimensions along with the specification assure that Barnoski et al.'s multimode transmission and, hence, maintaining phase relationships are not anticipated. The coupling between the fibers is dependent on a fused cladding-on-cladding and a typical coupling efficiency is disclosed as being nearly eight percent. Thus, by design or by consequence multimode and multiphase coupling at low efficiencies are inherent in the Barnoski et al. coupler.
Thus, there is a continuing need in the state of the art for a coupler of optical data which lends itself toward coupling single-mode, in-phase signals between small, single-mode conductors. The coupler must be capable of withstanding environmental abuses such as temperature changes and mechanical influences, and still effect a reliable transfer of energy between adjacent fibers.